FileCheck reads two files (one from standard input, and one
specified on the command line) and uses one to verify the other. This
behavior is particularly useful for the testsuite, which wants to verify that
the output of some tool (e.g. llc) contains the expected information
(for example, a movsd from esp or whatever is interesting). This is similar to
using grep, but it is optimized for matching multiple different
inputs in one file in a specific order.

The match-filename file specifies the file that contains the patterns to
match. The file to verify is read from standard input unless the
--input-file option is used.

FileCheck searches the contents of match-filename for patterns to
match. By default, these patterns are prefixed with “CHECK:”.
If you’d like to use a different prefix (e.g. because the same input
file is checking multiple different tool or options), the
--check-prefix argument allows you to specify one or more
prefixes to match. Multiple prefixes are useful for tests which might
change for different run options, but most lines remain the same.

By default, FileCheck allows matches of anywhere on a line. This
option will require all positive matches to cover an entire
line. Leading and trailing whitespace is ignored, unless
--strict-whitespace is also specified. (Note: negative
matches from CHECK-NOT are not affected by this option!)

Passing this option is equivalent to inserting {{^*}} or
{{^}} before, and {{*$}} or {{$}} after every positive
check pattern.

By default, FileCheck canonicalizes input horizontal whitespace (spaces and
tabs) which causes it to ignore these differences (a space will match a tab).
The --strict-whitespace argument disables this behavior. End-of-line
sequences are canonicalized to UNIX-style \n in all modes.

Adds implicit negative checks for the specified patterns between positive
checks. The option allows writing stricter tests without stuffing them with
CHECK-NOTs.

For example, “--implicit-check-notwarning:” can be useful when testing
diagnostic messages from tools that don’t have an option similar to clang-verify. With this option FileCheck will verify that input does not contain
warnings not covered by any CHECK: patterns.

FileCheck is typically used from LLVM regression tests, being invoked on the RUN
line of the test. A simple example of using FileCheck from a RUN line looks
like this:

; RUN: llvm-as < %s | llc -march=x86-64 | FileCheck %s

This syntax says to pipe the current file (“%s”) into llvm-as, pipe
that into llc, then pipe the output of llc into FileCheck. This
means that FileCheck will be verifying its standard input (the llc output)
against the filename argument specified (the original .ll file specified by
“%s”). To see how this works, let’s look at the rest of the .ll file
(after the RUN line):

Here you can see some “CHECK:” lines specified in comments. Now you can
see how the file is piped into llvm-as, then llc, and the machine code
output is what we are verifying. FileCheck checks the machine code output to
verify that it matches what the “CHECK:” lines specify.

The syntax of the “CHECK:” lines is very simple: they are fixed strings that
must occur in order. FileCheck defaults to ignoring horizontal whitespace
differences (e.g. a space is allowed to match a tab) but otherwise, the contents
of the “CHECK:” line is required to match some thing in the test file exactly.

One nice thing about FileCheck (compared to grep) is that it allows merging
test cases together into logical groups. For example, because the test above
is checking for the “sub1:” and “inc4:” labels, it will not match
unless there is a “subl” in between those labels. If it existed somewhere
else in the file, that would not count: “grepsubl” matches if “subl”
exists anywhere in the file.

The FileCheck -check-prefix option allows multiple test
configurations to be driven from one .ll file. This is useful in many
circumstances, for example, testing different architectural variants with
llc. Here’s a simple example:

Sometimes you want to match lines and would like to verify that matches
happen on exactly consecutive lines with no other lines in between them. In
this case, you can use “CHECK:” and “CHECK-NEXT:” directives to specify
this. If you specified a custom check prefix, just use “<PREFIX>-NEXT:”.
For example, something like this works as you’d expect:

Sometimes you want to match lines and would like to verify that matches happen
on the same line as the previous match. In this case, you can use “CHECK:”
and “CHECK-SAME:” directives to specify this. If you specified a custom
check prefix, just use “<PREFIX>-SAME:”.

“CHECK-SAME:” is particularly powerful in conjunction with “CHECK-NOT:”
(described below).

The “CHECK-NOT:” directive is used to verify that a string doesn’t occur
between two matches (or before the first match, or after the last match). For
example, to verify that a load is removed by a transformation, a test like this
can be used:

If it’s necessary to match strings that don’t occur in a strictly sequential
order, “CHECK-DAG:” could be used to verify them between two matches (or
before the first match, or after the last match). For example, clang emits
vtable globals in reverse order. Using CHECK-DAG:, we can keep the checks
in the natural order:

CHECK-NOT: directives could be mixed with CHECK-DAG: directives to
exclude strings between the surrounding CHECK-DAG: directives. As a result,
the surrounding CHECK-DAG: directives cannot be reordered, i.e. all
occurrences matching CHECK-DAG: before CHECK-NOT: must not fall behind
occurrences matching CHECK-DAG: after CHECK-NOT:. For example,

; CHECK-DAG: BEFORE; CHECK-NOT: NOT; CHECK-DAG: AFTER

This case will reject input strings where BEFORE occurs after AFTER.

With captured variables, CHECK-DAG: is able to match valid topological
orderings of a DAG with edges from the definition of a variable to its use.
It’s useful, e.g., when your test cases need to match different output
sequences from the instruction scheduler. For example,

While this can be very useful, it’s also dangerous, because in the case of
register sequence, you must have a strong order (read before write, copy before
use, etc). If the definition your test is looking for doesn’t match (because
of a bug in the compiler), it may match further away from the use, and mask
real bugs away.

In those cases, to enforce the order, use a non-DAG directive between DAG-blocks.

Sometimes in a file containing multiple tests divided into logical blocks, one
or more CHECK: directives may inadvertently succeed by matching lines in a
later block. While an error will usually eventually be generated, the check
flagged as causing the error may not actually bear any relationship to the
actual source of the problem.

In order to produce better error messages in these cases, the “CHECK-LABEL:”
directive can be used. It is treated identically to a normal CHECK
directive except that FileCheck makes an additional assumption that a line
matched by the directive cannot also be matched by any other check present in
match-filename; this is intended to be used for lines containing labels or
other unique identifiers. Conceptually, the presence of CHECK-LABEL divides
the input stream into separate blocks, each of which is processed independently,
preventing a CHECK: directive in one block matching a line in another block.
If --enable-var-scope is in effect, all local variables are cleared at the
beginning of the block.

The use of CHECK-LABEL: directives in this case ensures that the three
CHECK: directives only accept lines corresponding to the body of the
@C_ctor_base function, even if the patterns match lines found later in
the file. Furthermore, if one of these three CHECK: directives fail,
FileCheck will recover by continuing to the next block, allowing multiple test
failures to be detected in a single invocation.

There is no requirement that CHECK-LABEL: directives contain strings that
correspond to actual syntactic labels in a source or output language: they must
simply uniquely match a single line in the file being verified.

All FileCheck directives take a pattern to match.
For most uses of FileCheck, fixed string matching is perfectly sufficient. For
some things, a more flexible form of matching is desired. To support this,
FileCheck allows you to specify regular expressions in matching strings,
surrounded by double braces: {{yourregex}}. FileCheck implements a POSIX
regular expression matcher; it supports Extended POSIX regular expressions
(ERE). Because we want to use fixed string matching for a majority of what we
do, FileCheck has been designed to support mixing and matching fixed string
matching with regular expressions. This allows you to write things like this:

; CHECK: movhpd {{[0-9]+}}(%esp), {{%xmm[0-7]}}

In this case, any offset from the ESP register will be allowed, and any xmm
register will be allowed.

Because regular expressions are enclosed with double braces, they are
visually distinct, and you don’t need to use escape characters within the double
braces like you would in C. In the rare case that you want to match double
braces explicitly from the input, you can use something ugly like
{{[{][{]}} as your pattern.

It is often useful to match a pattern and then verify that it occurs again
later in the file. For codegen tests, this can be useful to allow any register,
but verify that that register is used consistently later. To do this,
FileCheck allows named variables to be defined and substituted into
patterns. Here is a simple example:

The first check line matches a regex %[a-z]+ and captures it into the
variable REGISTER. The second line verifies that whatever is in
REGISTER occurs later in the file after an “andw”. FileCheck
variable references are always contained in [[]] pairs, and their names can
be formed with the regex [a-zA-Z_][a-zA-Z0-9_]*. If a colon follows the name,
then it is a definition of the variable; otherwise, it is a use.

FileCheck variables can be defined multiple times, and uses always
get the latest value. Variables can also be used later on the same line they
were defined on. For example:

; CHECK: op [[REG:r[0-9]+]], [[REG]]

Can be useful if you want the operands of op to be the same register,
and don’t care exactly which register it is.

If --enable-var-scope is in effect, variables with names that
start with $ are considered to be global. All others variables are
local. All local variables get undefined at the beginning of each
CHECK-LABEL block. Global variables are not affected by CHECK-LABEL.
This makes it easier to ensure that individual tests are not affected
by variables set in preceding tests.

Sometimes there’s a need to verify output which refers line numbers of the
match file, e.g. when testing compiler diagnostics. This introduces a certain
fragility of the match file structure, as “CHECK:” lines contain absolute
line numbers in the same file, which have to be updated whenever line numbers
change due to text addition or deletion.

To support this case, FileCheck allows using [[@LINE]],
[[@LINE+<offset>]], [[@LINE-<offset>]] expressions in patterns. These
expressions expand to a number of the line where a pattern is located (with an
optional integer offset).

This way match patterns can be put near the relevant test lines and include
relative line number references, for example: